Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual Surface tem um oponente de Oleksandra Kravets?
 ###Tabelas: CREATE TABLE characters_casts_and_recordings (characters_casts_and_recordings_id AUTO_INCREMENT PRIMARY KEY, Character VARCHAR, Original Broadway performer VARCHAR, Original Australian performer VARCHAR, Original West End performer VARCHAR, 2005 World AIDS Day Benefit Dream Cast VARCHAR) CREATE TABLE doubles_25_12–13 (doubles_25_12–13_id AUTO_INCREMENT PRIMARY KEY, Outcome VARCHAR, Date VARCHAR, Tournament VARCHAR, Surface VARCHAR, Partner VARCHAR, Opponents in the final VARCHAR, Score VARCHAR) CREATE TABLE itf_singles_finals (itf_singles_finals_id AUTO_INCREMENT PRIMARY KEY, Outcome VARCHAR, Date VARCHAR, Tournament VARCHAR, Surface VARCHAR, Opponent VARCHAR, Score VARCHAR) CREATE TABLE ships_of_the_great_eastern_railway (ships_of_the_great_eastern_railway_id AUTO_INCREMENT PRIMARY KEY, Ships name VARCHAR, Entered service VARCHAR, Ended service VARCHAR, Gross Tonnage INT, Type of Vessel VARCHAR) CREATE TABLE list_of_sponsored_cosponsored_legislation (list_of_sponsored_cosponsored_legislation_id AUTO_INCREMENT PRIMARY KEY, Years covered VARCHAR, All bills sponsored INT, All amendments sponsored INT, All bills cosponsored INT, All amendments cosponsored INT, Original bills cosponsored INT, Original amendments cosponsored INT, Bill support withdrawn INT, Amendment support withdrawn INT)  
SELECT Surface FROM itf_singles_finals WHERE Opponent = oleksandra kravets